A Passive Optical Network (PON) includes an Optical Distribution Network (ODN), which contains no electronic device or electronic power source and is entirely composed of passive devices such as optical splitters. A PON includes an Optical Line Terminal (OLT) installed in a central office and a batch of Optical Network Units (ONUs) installed at the customer premise. Three PON technologies are provided, Asynchronous Transfer Mode Passive Optical Network (APON), Ethernet Passive Optical Network (EPON) and Gigabit Passive Optical Network (GPON). EPON and GPON also evolve to the next generation PON (xPON).
Taking GPON as an example, the GPON protocol stack is illustrated in FIG. 1. The GPON protocol stack includes three layers, and they are briefly introduced in a down to top sequence.
One is the GPON Physical Medium Dependent (GPM) layer, which is responsible for the transmission of GPON Transmission Convergence (GTC) layer frames on optical fibers. It transmits optical signals from the optical fibers to the PON Media Access Control (MAC) layer for data processing and converts data signals received from the PON MAC layer into optical signals.
A second layer is the MAC layer. For GPON, the MAC layer is a GTC layer, which includes two sub-layers:
(a) TC adapter sublayer
The TC adapter sub-layer is responsible for fragmenting service data received from an Asynchronous Transfer Mode (ATM) client into ATM cells and fragmenting service data received from a GPON Encapsulation Method (GEM) client into GEM data blocks; the TC adapter sub-layer is also responsible for assembling ATM cells or GEM data blocks in a GTC frame to appropriate service data.
(b) GTC framing sub-layer
The GTC framing sub-layer is responsible for assembling GTC TC frames. Specifically, the GTC framing sub-layer adds a GTC TC frame header before an ATM cell or a GEM data block according to control information of Physical Layer Operation, Administration and Maintenance (PLOAM) to create a complete GTC TC frame and send the frame to the GPM layer; the GTC framing sub-layer is also responsible for removing frame header information from a GTC TC frame received from the GPM layer and sending the frame with the frame header information removed to the TC adapter sub-layer for processing.
The GPON also has a third layer, which includes the ATM client, GEM client and the following units:
(1) PLOAM: responsible for functions like operation, administration and maintenance at the PON physical layer; and
(2) ONU Management and Control Interface (OMCI): the OLT controls an Optical Network Terminal (ONT) via the OMCI; like common service data, OMCI data can be encapsulated to ATM cells or GEM data blocks for transmission.
IEEE 1588 is the Precision Time Protocol (PTP) of a system for network measurement and control, and implements synchronization of the slave clock of an ONT (client device) with the master clock of the main control device by sending/receiving clock packets. The principle of the IEEE 1588 PTP protocol is described as follows: Based on the most precise time when the synchronization packets are sent and received, each slave clock exchanges synchronization packets with the master clock to achieve synchronization with the master clock.
The synchronization process includes two stages: offset measurement stage and delay measurement stage.
FIG. 2A illustrates the offset measurement stage where the master clock broadcasts two messages to all nodes on the network:
1. sync message: denoting desired time for sending the message
2. follow-up message: denoting actual time for sending the message
The sync messages are sent automatically at given intervals. The follow-up messages are employed to calculate the transmission delay caused by the local protocol when packets are sent. The master clock sends determined sync messages at regular intervals (generally once two seconds). The sync message contains a time stamp, which describes precisely the desired time the packet is sent. Assume that the time of the master clock before synchronization is Tm=128 s and that the slave clock time is Ts=111 s. The master clock measures that the precise sending time is Tm1 and the slave clock measures that the precise receiving time is Ts1. Because the sync message contains the desired sending time instead of the actual sending time, the master clock sends a follow-up message which contains a time stamp that records precisely the actual sending time Tm1 of the sync message. Thus, the slave clock can calculate the offset between the slave clock and the master clock according to the actual sending time in the follow-up message and the actual receiving time of the receiver:Offset=Ts1−Tm1−Delay=111.75−128.5−0=16.75 s
The “delay” above means the transmission delay between the master clock and the slave clock and will be measured in the following measurement stage. At the current stage, the delay is unknown and assumed to be 0.
At the offset measurement stage, Adjust Time can be obtained and the slave clock is adjusted to:Adjust Time=Ts−Offset
The second stage is the delay measurement stage as shown in FIG. 2B.
The delay measurement stage measures the delay caused by network transmission. The measurement is achieved through exchange of the following messages between the master clock and the slave clock:
1. The slave clock sends a Delay Request message, informing the master clock, “I send the Delay Request message at this moment.”
2. The master clock sends a Delay Response message, informing the slave clock, “I receive your Delay Request message at this moment.”
The slave clock sends the Delay Request at Ts3 130.75 s after receiving the sync message. The master clock sends the Delay Response to the slave clock after receiving the Delay Request and marks the precise receiving time Tm3 131.25 s in the Delay Response. Thus, the slave clock can calculate the accurate network delay.Delay=(Tm3−Ts3)/2=(131.25−130.75)/2=0.25
IEEE 802.3 defines the basic structure of an Ethernet frame, including: preamble, Start Frame Delimiter (SFD), destination address, source address, length field, data field, and frame check sequence.
As shown in FIG. 3, the preamble consists of 8 bits of alternated 1s and 0s. The SFD includes 8 bits where the first 6 bits are alternated 1s and 0s and the last 2 bits are “1, 1” indicating the start of the frame to the receiver. Following the two bits are the actual fields of the frame.
In an Ethernet, all clock packets defined by IEEE 1588/1588v2 are transmitted in the form of IP multicast packets. The packet time stamp generating point for determining the time a clock packet is transmitted or received is located at the last bit of the SFD.
During the implementation of the present invention, the inventor finds at least the following weaknesses in the prior art: When Ethernet data is encapsulated to GEM (that is, when the “Ethernet over GEM” mode is employed), each Ethernet frame is mapped into a GEM frame. As shown in FIG. 4, the GEM frame does not include the preamble and SFD, and the destination address, source address, length field, data field, and frame check sequence field of the Ethernet frame are directly mapped into the GEM payload for transmission. The GEM frame is automatically encapsulated with the GEM frame header which includes four parts: Payload Length Indicator (PLI, 12 bits), Port ID (12 bits), Payload Type Indicator (PTI, 3 bits), and Header Error Control (HEC, 13 bits).
In case of Ethernet over GEM mode, the Ethernet time stamp generating point required for sending IEEE 1588/1588v2 clock packets is lost. As a result, the time synchronization method defined by IEEE 1588/1588v2 is not supported in “Ethernet over GEM” mode.